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re: heat pumps
5 may 2005
jeff wisnia   wrote:

>i'm still trying to get my brain to accept that heat pumps can put out 2 
>to 3 times as much heat as the kilowatt hours they use...

that  is  sort of magical...

>i keep trying to find a description of what the limitations are...

the basic limit comes from carnot, who said the maximum possible heating
cop (energyout/energyin) with absolute heat source and sink temps tcold and
thot is thot/(thot-tcold.) for example, if thot = 70 f (= 460+70 rankine)
and tcold = 50, copmax = (460+70)/(70-50) = 26.5 :-)

>and why that performance number is what it is and not something different.

friction and heat loss make it closer to 3, and electric motors with 80%
vs 100% efficiencies, and finite-size heat exchangers with less than 100%
efficiency, which have the effect of raising thot and lowering tcold. fans
and pumps make heat exchangers more efficient, but they also use energy.
if house air is 70 f and the hot fluid is 80 vs 70 f and it's 50 outdoors
but the cold fluid is 40, copmax = (460+80)/(80-40) = 13.5.

and the fluid must be economical and safe, and real heat pumps waste energy
in throttling vs perfect fluid expansion. 

nick




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